I’ll mark this for the developer to take a look at if he has time.
I’d call this a feature request and not a bug. Looking at the code it appears to be working as intended.
Before attempting to write a file ACF does not test to see if the folder exists and if it’s writable separately, it test to see if it’s writable which has a side effect of also testing if it exists.
The getting started section of this page gives information on setting up the folder which includes the permissions that are required http://www.advancedcustomfields.com/resources/local-json/#getting-started
